1. Field of the Invention
The invention relates in general to a sigma-delta modulator, and more particularly to a sigma-delta modulator enabling a quantizer to utilize an independent reference voltage.
2. Description of the Related Art
A sigma-delta modulator has the advantages of high resolution, simpler circuit, no additional trimming circuit required, and higher tolerance to circuit element modifications.
FIG. 1 is a block diagram showing a conventional sigma-delta modulator 100. Referring to FIG. 1, the sigma-delta modulator 100 includes an adder 110, a low-pass filter 120, a quantizer 130 and a digital-to-analog converter (D/A converter) 140. The adder 110 receives an input signal X1 and an analog signal A11 and subtracts the analog signal A11 from the input signal X1 to output a processed signal S11. The low-pass filter 120 receives the processed signal to output a filtered signal S12. The quantizer 130 receives the filtered signal S12 to generate an output signal Y1. The digital-to-analog converter 140 generates the analog signal A11 according to the output signal Y1 and outputs the analog signal A11 to the adder 110.
In an advanced application of the sigma-delta modulator, the low-pass filter 120 includes multiple integrators and generates multiple integration signals, which are summated by the adder of the low-pass filter 120 to generate the filtered signal S12.
However, the adder of the low-pass filter 120 has to take in several signals, and the equivalent loading capacitor thereof usually has to be much greater than that of the integrator. If the above-mentioned sigma-delta modulator is applied to a high-resolution audio product, the reference voltages in the circuit produce an extremely large potential difference, such as the potential difference between a voltage source Vdd and a grounding voltage Gnd. This enables the maximum output swing of the adder to approach the potential difference between the voltage source Vdd and the grounding voltage Gnd. In this case, the power-consumption of the adder circuit is considerably large. In addition, the high-speed broadband sigma-delta modulator also causes large current consumption under the requirement of the high slew rate.